Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on October 27, 2016, 11:08:56 am
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This PT has no CT tap for the filaments both 5V and 6.3V, 100 ohm resistors are used for the CT on the 6.3v filaments but shouldn't two 100 ohms also be used for the 5v filaments?
The 6.3 v filaments are A/C, is the 5v, A/C too (I think it's D/C and no CT tap is needed but not sure?
https://www.tubesandmore.com/sites/default/files/associated_files/p-t290ax.pdf (https://www.tubesandmore.com/sites/default/files/associated_files/p-t290ax.pdf)
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but shouldn't two 100 ohms also be used for the 5v filaments?
NEVER! Think about it. There is 5VAC between those yellow wires. But the cathode of the rectifier tube is also connected to the filaments. So there will be B+ voltage floating on those yellow wires too. If you connect 100Ω resistors to ground they will pop as soon as the rectifier tube warms up.
The 6.3 v filaments are A/C, is the 5v, A/C too (I think it's D/C and no CT tap is needed but not sure?
Transformers put out AC volts only.
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It is never wrong to use the dual 100 ohm resistors, even if the tranny (talking about the 6.3 winding of the tranny....right?) has a real CT. Because they provide a certain piece of unique protection against a heater to cathode short in the output tubes. Arguably, it is superior to use them. A H-K short in the outputs is very, very rare but not never.
The 6.3 winding lighting up the tubes, that has one and only one function: To light up the tubes. In 99+% of cases, the CT of such a winding is not precisely in the center of the winding and the effect of this is to induce AC hum into the SIGNAL path. 99% of tube audio amps need a 6.3 heater CT (either CT or real) or they will hum like a brontosaurus.
The 5V winding on the rectifier tube has TWO functions. One is to light up the tube. The other is to serve as/be at the same voltage as the cathode in the HV rectifier circuit. Unless it is not working right, the action of the rectifier does not find its way into the audio signal path. Even in an indirectly heated rectifier like a GZ34 or a 5V4 the heater (or filament = proper name for 5Y3/5U4) will be at full B+ volts. Actually, it has full on B+, NASTY B+ with huge ripple on it.
I am not sure what the effect of placing 100 ohm resistors on the 5V winding would be. Sluckey said they would probably blow and they probably would, but I actually doubt that because even though the entire heater/filament is raised high, there is only 5 volts across the heater wires. Ergo, 5 volts / 200 ohms = .025 amps. .025 amps * .025 amps * 200 ohms (in series) = .125 watts. Now...if you GROUNDED the middle of those resistors, they would absolutely pop, with extreme prejudice! That would be B+ to ground through a 50 ohm resistor (the two in parallel) = 400 / 50 = 8 watts, one would smoke first and the other in very short order.
Don't do it!
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Thanks for the explanation, sure makes sense, I should have known that but I do now. For some reason I though 5 volt rectifier tubes were heated with dc, but that doesn't make sense as Sluckey mentioned. A PT is either putting out ac or dc not both.
What if one supplied DC to the primary of a PT, would it then put out AC on all the sec's?
thanks again,
al
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What if one supplied DC to the primary of a PT, would it then put out AC on all the sec's?
No. It would put out nothing. Transformers only work with AC.
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Thanks Sluckey, at least I'm thinking, "wrong" but thinking...? :icon_biggrin:
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That's always a good thing! :wink:
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Oh by the way, if you are thinking on using the dual 100 ohms, DON'T use the wire CT. One or the other, not both.
It would probably not hurt anything, just don't do it.
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Oh by the way, if you are thinking on using the dual 100 ohms, DON'T use the wire CT. One or the other, not both.
It would probably not hurt anything, just don't do it.
I only use the 100 ohm CT if the PT I'm using doesn't have a CT for the filaments, of course only for the 6.3v and not the 5v filament, :icon_biggrin:
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There you go thinkin' again. :icon_biggrin: